According to the World Health Organization (WHO), Mycobacterium tuberculosis (Mtb or M. tuberculosis), a contagious and airborne bacterial pathogen, infects one third of the world population. As noted by the Centers for Disease Control and Prevention (CDC), tuberculosis (TB) is caused by exposure to Mtb which usually attack the lungs. However, TB bacteria may attack any part of the body such as the kidney, spine, and brain. If not treated properly, TB disease can be fatal. In 2012 alone, there were 8.6 million new human cases and 1.3 million deaths caused by TB. However, not every human infected with TB bacteria becomes ill. As a result, two TB-related conditions exist: latent TB infection and TB disease.
Latent TB infection involves asymptomatic exposure to TB bacteria. In most people who breathe in TB bacteria and become infected, the body is able to fight the bacteria to stop them from growing. People with latent TB infection do not feel sick and do not have any symptoms. People with latent TB infection are not infectious and cannot spread TB bacteria to others.
However, if TB bacteria become active in the body and multiply, the person will go from having latent TB infection to being sick with TB disease. TB bacteria become active if the immune system is compromised and thus unable to counter Mtb growth. When TB bacteria are active (multiplying), this is called TB disease. People with TB disease are sick and infectious. This may lead to the development of additional latent TB exposure and active TB disease in others.
Many people who have latent TB infection never develop TB disease. Some people develop TB disease soon after becoming infected (within weeks) before their immune system can fight the TB bacteria. Other people may get sick years later when their immune system becomes weak for another reason. For people whose immune systems are weak, especially those with HIV infection, the risk of developing TB disease is much higher than for people with normal immune systems.
Thus, TB-HIV co-infection and the emergence of multi-drug resistant TB are posing an unprecedented threat to public health. Failure to control the TB epidemic is largely due to lack of effective vaccines against TB. The current and the only approved TB vaccine is Mycobacterium bovis bacillus Calmette-Guerin (BCG), which is an attenuated strain of M. bovis. BCG has been used for nearly 100 years and is the most widely used TB vaccine worldwide. While BCG provides protection against TB in newborns, it does not prevent the establishment of latent TB or reactivation of pulmonary TB in adults. There remains a need for additional vaccine compositions and related methods of use and treatment for TB.